/Engine/Source/Runtime/PakFile/Public/IPlatformFilePak.h
Finds all the files within the directory tree, with optional file extension filter
FoundFiles All the files that matched the optional FileExtension filter, or all files if none was specified.
virtual void FindFilesRecursively ( [TArray](API\Runtime\Core\Containers\TArray)< [FString](API\Runtime\Core\Containers\FString) > & FoundFiles, const TCHAR * Directory, const TCHAR * FileExtension )